1. Field of the Invention
This invention relates to a catalyst for electrode (hereinafter referred to as "electrocatalyst") for acid electrolyte type fuel cells such as phosphoric acid type fuel cells or proton-exchange membrane type fuel cells.
2. Description of the Prior Art
Catalysts comprising a conductive carbon powder carrier on which platinum (Pt) is dispersedly supported and having a corrosion resistance to phosphoric acid heated at a high temperature of about 200.degree. C. are conventionally used as electrocatalysts for the phosphoric acid type fuel cells. Furthermore, in recent years, catalysts comprising a conductive carbon powder on which an alloy of platinum with a base metal element such as gallium, titanium, vanadium, chromium, iron, cobalt, nickel or copper is supported have been developed in order to improve the activity of oxygen reduction reaction in the cathode (U.S. Pat. Nos. 4,186110 and 4,447,506, Japanese Pre-examination Patent Publication (kokai) Nos. 60-7941, 61-8851 and 62-163746, etc.). Meanwhile, as electrocatalysts for proton-exchange membrane type fuel cells, platinum black catalysts have been used from old times, but catalysts comprising a conductive carbon powder on which platinum is highly dispersedly supported are now put into use so that the amount of platinum used can be reduced. Also in such catalysts, researches are made on catalysts formed of an alloy with a base metal so that the oxygen reduction activity of the cathode can be improved G. Tamizhmani & G. A. Capuano, J. Electrochem. Soc., Vo.141, No.4 (1994)!.
Such platinum alloy-supported catalysts are usually produced in the following way. First, platinum is supported on a conductive carbon powder to prepare a platinum-supported carbon powder (a precursor), and then this precursor platinum-supported carbon powder is added in an aqueous solution of a compound such as a salt of base metal element to fix the base metal component on the platinum-supported carbon by the aid of an alkali such as sodium-hydroxide or ammonia or a reducing agent such as hydrazine or formalin, followed by treatment in a stream of nitrogen or in a stream of hydrogen at a high temperature of 800.degree. C. to 1,000.degree. C. to alloy the base metal and platinum.
Such platinum alloy-supported catalysts have an improved oxygen reduction activity compared with the precursor platinum-supported carbon powder and hence are known to be greatly improved in current-voltage characteristics (I-V characteristics; the same applies hereinafter) when used as cathode catalysts for phosphoric acid type fuel cells.
Japanese Pre-examination Patent Publication (kokai) No. 3-127459 reports that in the production of such an alloy-supported catalyst all the base metal used is not completely made into an alloy and is partly deposited on and adheres to the carbon surface, and it is proposed to remove free base metal by washing the catalyst with an aqueous dilute acid solution to thereby suppress the wetting of the catalyst to the electrolyte so that the cell can have a longer lifetime.
Advantages of skeleton catalysts whose representative is Raney nickel are also known from old times. Platinum skeleton catalyst electrodes prepared by dissolving the base metal component out of an electrode formed of an alloy of platinum with a base metal, not supported on a carrier, were studied in the 1960's as electrodes for fuel cells (U.S. Pat. No. 3,429,75). Meanwhile, a platinum-supported skeleton catalyst prepared by removing Fe from a Pt-Fe alloy-supported catalyst (U.S. Pat. No. 4,187,200) is known to exhibit a high activity in hydrogenation reaction, but there is no disclosure as to whether it is useful as an electrocatalyst.
Under operation conditions of fuel cells in which an acid electrolyte and oxygen are present together, not only the base metal not alloyed with platinum but also the base metal in platinum-base metal alloy crystals can not avoid gradually dissolving in the electrolyte. The base metal having dissolved in the electrolyte is either reduced at a portion having a more basic potential in the cell to become deposited there, or crystallized because of a lowering of solubility at a lower-temperature portion to become deposited, thus there is a possibility of clogging pores of a gas diffusion electrode. Especially in the case of proton-exchange membrane type fuel cells, there is also a possibility that base metal ions react with the electrolyte to cause a lowering of the conductivity of the electrolyte. Hence, in order to make fuel cells have a longer lifetime, it is desired to make the base metal less dissolve out of the alloy.
Meanwhile, in the base metal/platinum alloy-supported catalysts, it is usually indispensable to alloy the materials at a high temperature in the step of preparation. As a result of this high-temperature treatment, the platinum is alloyed with the base metal and at the same time may cause crystal growth. Hence, it is not always easy to prepare a platinum alloy catalyst having, e.g., an average crystal particle size not larger than 50 A (angstrom, 1 A=0.1 nm; the same applies hereinafter). Accordingly, it is sought to provide as an electrocatalyst for fuel cells a platinum-supported catalyst that retains crystal forms of an alloy highly active in oxygen reduction and yet has platinum alloy particles made finer and also containing less base metal component.